Our invention relates to privacy arrangements in communication systems and, more particularly, to privacy systems adapted to encrypt a signal by reordering time segments of the signal.
In telephone and other types of communication systems, it is often necessary to render a signal being transmitted unintelligible to assure privacy. Such secret communication systems have generally been restricted to selected communication channels over which secret messages are expected to be sent. Privacy is also desirable where messages of a nonconfidential nature are transmitted over a common communication path that is easily accessible to third parties. Thus, privacy arrangements are applicable to radio communication systems such as mobile telephone where interception is readily accomplished, and also to wire communication systems such as those utilizing time division switching where connections to a common time division bus system may result in interception of a speech signal by means of crosstalk between time channels or otherwise.
One known privacy method is operative to divide a speech or other intelligence signal from a source into successive time segments. The segments are rearranged to render the signal unintelligible, and the time-scrambled signal is transmitted. An inverse rearrangement of the scrambled signal at the destination point reconstructs the original intelligence signal. Signal segment rearrangement in a predetermined repetitive order may, of course, be understood or readily decrypted by a third party having access to the communication path since the encrypted signal is closely correlated with the original intelligence signal. Consequently, signal element rearrangement has been accomplished by pseudo-random schemes or by schemes involving a complex, nonuniform permutation of signal segments. Such schemes result in an encrypted signal which is uncorrelated with the original intelligence signal. While such pseudo-random or nonuniform permutation schemes are widely used, the apparatus for implementing these schemes and the keys used to encipher and decipher the permutated signal are relatively complex, owing to the long-term nonrepeatability of the permutations to produce the uncorrelated scrambled signal. Where, however, privacy devices are attached to each terminal of a large communication system, such as in mobile telephone or a time division PBX, the complexity of the pseudo-random or nonuniform permutation encrypting, decrypting, and keying apparatus renders the privacy feature uneconomical. Thus, in order to provide privacy to all subscribers served by a large communication system, it is advantageous to utilize a relatively simple uniform permutation arrangement that retains the described encryption characteristics.
It is an object of the invention to provide an improved privacy system which avoids complicated scrambling arrangements.